OFFSET
1,2
COMMENTS
Are 1 and 36 the only terms that are also triangular numbers?
No other triangular terms up to A000217(10^8). - Michel Marcus, Mar 01 2020
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..48
EXAMPLE
5 is a term since sigma(5) = 6 and sigma(5) - 5 = 1 are both triangular numbers.
MATHEMATICA
triQ[n_] := IntegerQ @ Sqrt[8*n+1]; Select[Range[10^5], triQ[(s = DivisorSigma[1, #])] && triQ[s - #] &]
PROG
(PARI) isok(k) = my(s=sigma(k)); ispolygonal(s, 3) && ispolygonal(s-k, 3); \\ Michel Marcus, Feb 29 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Feb 28 2020
STATUS
approved